Wash the Jersey Royals and put into a large pan of salted water and bring to the boil. Cook for about eight to ten minutes, until tender. Drain and set aside.
Heat the grill to maximum temperature, season the steaks with salt and cover with the peppercorns. Grill for about 4 minutes each side for medium, 3 minutes each side for medium-rare and 6 minutes for well done. Leave to rest for somewhere warm for 5 minutes.
Finely slice the spring onions and stir into the yogurt with the herbs. Taste and adjust the seasoning.
Add the potatoes to the yogurt, stir well and lightly crush with a fork.
Serve the potatoes with the steak.